1. Field of the Invention
The present invention relates to an energy-trap type piezoelectric filter and, more particularly, to a piezoelectric filter having improved characteristics resulting from an improved polarized structure of a piezoelectric substrate.
2. Description of the Related Art
Piezoelectric filters are widely used as intermediate frequency stage band filters in mobile communication devices such as FM receivers and portable telephones. FIGS. 7A and 7B show a piezoelectric filter 51 as an example of this kind of filter.
The piezoelectric filter 51 includes a piezoelectric ceramic substrate 52 in the form of a rectangular plate. The entire area of the piezoelectric ceramic substrate 52 other than the area within the broken line A is polarized in the direction of thickness.
Energy-trap type filter sections 53 and 54 and a capacitor 55 are arranged on the piezoelectric ceramic substrate 52. The filter section 53 is constructed by disposing resonant electrodes 53a and 53b on the upper surface of the piezoelectric substrate 52 and by providing a common electrode 53c on the lower surface of the piezoelectric substrate 52. The resonant electrodes 53a and 53b on the upper surface of the piezoelectric substrate 52 are opposed to each other with a predetermined gap provided therebetween. Also, the resonant electrodes 53a and 53b and the common electrode 53c are disposed opposite to each other on opposite surfaces of the piezoelectric substrate 52.
The filter section 54 is constructed in the same manner as the filter section 53, and has resonant electrodes 54a and 54b and a common electrode 54c.
To form a capacitor 55, a capacitive electrode 55a is arranged on the upper surface of the central unpolarized region of the piezoelectric substrate 52 indicated by the broken line A, and a capacitive electrode 55b is provided on the lower surface of the unpolarized region. The capacitive electrodes 55a and 55b are disposed opposite to each other on opposite surfaces of the piezoelectric substrate 52.
A lead-out electrode 56 is arranged on the upper surface of the piezoelectric substrate 52 along one end surface 52a thereof. The lead-out electrode 56 is electrically connected to the resonant electrode 53a. The resonant electrode 53b is electrically connected to the capacitive electrode 55a. The capacitive electrode 55a is connected to the resonant electrode 54b. The resonant electrode 54a is connected to a lead-out electrode 57, which is arranged on the upper surface of the piezoelectric substrate 52 along another end surface 52b thereof.
On the lower surface of the piezoelectric substrate 52, the capacitive electrode 55b is connected to the common electrodes 53c and 54c.
Therefore, the piezoelectric filter 51 can be operated as a dual mode piezoelectric filter having the circuit shown in FIG. 8 by using the lead-out electrodes 56 and 57 as input and output electrodes and by connecting the capacitive electrodes 55b and the common electrodes 53c and 54c disposed on the lower surface of the piezoelectric substrate 52 to a ground potential point.
The unpolarized area of the piezoelectric substrate 52 indicated by broken line A, consisting of the portion on which the capacitive electrodes 55a and 55b are formed and a vicinity of the same, is left unpolarized in order to set tan .delta. of the capacitor 55 to a small value.
In the piezoelectric filter 51, polarization of the portions of the piezoelectric substrate 52 constituting the filter sections 53 and 54 and other adjacent portions is necessary for excitation of the piezoelectric substrate 52 and for smooth attenuation of vibration at the filter sections 53 and 54. On the other hand, it is desirable to leave the portion constituting the capacitor 55 unpolarized so that tan .delta. of the capacitor 55 can be small, as mentioned above.
In the piezoelectric filter 51, however, the portions of the piezoelectric substrates on which the lead-out electrodes 56 and 57 are provided are also polarized and as a result, it is possible, depending upon the electrode layout, that undesirable and unnecessary vibration will result in a deterioration in filter characteristic.
Therefore, it is also desirable to leave the portion of the piezoelectric ceramic substrate 52 where the lead-out electrodes 56 and 57 are located unpolarized. If so, the unpolarized portion must be located in a region where the vibration by the filter sections 53 and 54 does not reach. Thus, the unpolarized portion can not be extended near to the portion of the piezoelectric ceramic substrate 52 where the filter sections 53 and 54 are located.